Serpentine belt tensioner is jumping and acting weird

Okay:
- Tensioner is good
- Original (too big) belt was 81 5/8" (Gates K070810)
- Tried next two sizes (down) of belt: 80 3/4" (K070801) and 80" (K070795)
- 80 3/4" went on 'a little too easy' so went with 80"
- 80" definitely snug fit when brand new and no rattle... for a while. There was very little range left on the tensioner with this belt. Meaning it was almost all the way in and would therefore have nearly its entire travel to stretch the belt out.
- However, after running engine for a while and doing some tuning, eventually the tensioner starter bouncing again. First, a little, then kind of violently, then started to kill the belt. The inner-most rib came apart and shot a piece out of the engine bay.
- I should note that although there may be discrepancies between rib count (will check when I get to the shop later), the old belt was exactly the same rib count (7) and never had a problem.

Today's experiment will be to run the 80 3/4" belt and add the washers as spacers underneath the AC delete pulley if/as needed.

I bought this car, a forgotten and neglected 1973 Dart Sport 340 in March to replace my poor wrecked Duster (Death of Huey the Hooptie) and bracket race with it. I tore it down to the shell and fully rebuilt it over the last few months with as much of the Duster as I could.

I'm THIS CLOSE to being able to hit the track tomorrow afternoon (we only have amateur bracket races on some Wednesdays at Sonoma Raceway). I've got an alignment appointment tomorrow and just need to get this belt thing under control! Why is it always the weird little things?!?